There has recently been used a honeycomb structure as a filter to trap particulate matter (DPF) discharged from a diesel engine to take measures against automobile exhaust gas regulations which become more strict every year. There has also been used a honeycomb structure with a catalyst carried thereon in order to remove nitrogen oxides, sulfur oxides, hydrogen chloride, hydrocarbon, carbon monoxide, and the like, contained in automobile exhaust gas.
Such a honeycomb structure is required to have an enlarged pore diameter and an increased porosity for the purpose of a pressure loss reduction, or the like. A bigger honeycomb structure is also required for purifying exhaust gas from heavy vehicles.
However, a mean pore diameter of a honeycomb structure is increased in a conventional method for producing a honeycomb structure, there is a problem of deterioration in trapping efficiency, for example, when the honeycomb structure is used as a DPF. This problem tends to worsen when an attempt to increase its porosity or enlarge its size is made. In addition, there is a problem of deterioration in isostatic strength and thermal shock resistance in accordance with increase in porosity or size enlargement.
There has been disclosed control of a temperature rise rate from 400 to 1200° C. within a predetermined range in a firing step in a conventional method for producing a honeycomb structure for the purpose of enhancing its isostatic strength and reducing a pressure loss (see, e.g., Patent Document 1). In addition, there has been disclosed control of a temperature rise rate from 1100 to 1200° C. to be 60° C./hr or less and from 1200 to 1300° C. to be 80° C./hr or more for the purpose of obtaining a honeycomb structure having high percentage of water absorption and low thermal expansion coefficient (see, e.g., Patent Document 2).
Further, there has been disclosed control of a temperature rise rate at below 1100° C. to be 250° C./hr and at 1100° C. or more to be 30 to 300° C./hr with using a raw material having a predetermined particle diameter (see, e.g., Patent Document 3).
However, in such conventional methods, it is difficult to inhibit the lowering of trapping efficiency in accordance with enlargement of a mean pore diameter of a honeycomb structure in the case that a honeycomb structure is produced as a filter for a DPF, or the like.    Patent Document 1: Japanese Patent Application No. 2003-277162    Patent Document 2: Japanese Patent No. 2981034    Patent Document 3: Japanese Patent Publication No. 57-28390